1. Field of the Invention
The invention relates generally to non-volatile magnetic memory and particularly to non-uniform switching of non-volatile magnetic based memory.
2. Description of the Prior Art
Computers conventionally use rotating magnetic media, such as hard disk drives (HDDs), for data storage. Though widely used and commonly accepted, such media suffer from a variety of deficiencies, such as access latency, higher power dissipation, large physical size and inability to withstand any physical shock. Thus, there is a need for a new type of storage device devoid of such drawbacks.
There has been an extensive effort in development of alternative technologies such as Ovanic Ram (or phase-change memory), Ferromagnetic Ram (FeRAM), Magnetic Ram (MRAM), Nanochip, and others to replace memories used in current designs such as DRAM, SRAM, EEPROM/NOR flash, NAND flash and HDD in one form or another. Although these various memory/storage technologies have created many challenges, there have been advances made in this field in recent years. MRAM has exceptional advantage when compared to other memory technologies under development in the aspects of speed, write endurance and non-volatility.
Perpendicular MRAM is particularly noteworthy because of its adaptability to sub-30 nano meters (nm) size and high density. However, thermal stability has been a continued problem faced in the design of perpendicular MRAM and is described by thermal stability factor, Δ, described as follows:Δ=KuV/kBT   Eq. (1)
where “Ku” is the perpendicular anisotropy energy density of the storage magnetic layer of the MRAM, “V” is the volume of the storage magnetic layer, “kB” is the Boltzmann constant, and “T” is the absolute temperature (in Kelvin).
This factor inevitably reduces at a given anisotropy energy of the storage magnetic layer resulting in the thermal stability of each bit decreasing. For MRAM applications using extremely high data density, for example dynamic random access memory (DRAM) type of applications, where speed and data capacity are key parameters, lower thermal stability of the data bits may be tolerable, or may be mitigated with reasonable amounts of error correction coding (ECC) to make the overall design function in the targeted regime of application. Perpendicular MRAM currently has a critical dimension of approximately 30 nano meters (nm) heading toward 10 nm.
Applications of MRAM generally include a reference MRAM data bit, which provides a reference resistance for comparing the reference bits to the MRAM data bits to indicate whether or not the data bits are in high resistance or low resistance state. The reference bit is desired to be made of identical MRAM cell structure as that of the data bit because it simplifies both the fabrication process and the circuit design than the case where the reference bit is made of a pure resistor without a MTJ structure.
With MRAM reference bit being identical to a data bit, the reference bit has the same low thermal stability problem as indicated hereinabove. The ECC is not correcting reference bit errors. Rather, a special data refreshing and assurance mechanism may be needed to make sure the reference bit is always in the correct state before any read operation on the data bits, which is costly both in design and in operation. Additionally, such refresh mechanism may slow down the operation speed of the device considerably and make the device not usable in high data rate applications.
What is needed is a perpendicular MRAM that has suitably high thermal stability characteristics in the reference bits allowing it to remain stable with high confidence during long-term operations without special refresh mechanisms required after initialization. Meanwhile, the data bits may still possess a comparatively lower thermal stability for fast and low power write operation.